Why an EV Battery Dry Room Is Designed Around Dew Point, Not Relative Humidity
A battery dry room design starts from a single number, and it is not a humidity percentage. Lithium cell materials react with water vapour. Moisture picked up while electrodes are handled, stacked and filled does not announce itself on the line. It shows up later as gas generation, as lost capacity, as a shortened service life, and in the worst case as a safety problem in a finished cell.
That is why dry rooms are specified in dew point. Relative humidity is a ratio, and it moves every time the room temperature moves. Dew point is an absolute measure of how much water the air is actually carrying, so it is the only honest way to write the requirement down. This room had to hold -25 C, which is a small fraction of the moisture in ordinary conditioned air.
Reaching it is an accounting exercise before it is an equipment exercise. Every person in the room, every door opening, every material transfer, the envelope itself and all the make-up air are moisture sources working against the setpoint. Each one has to be quantified and added up, because the dehumidification has to beat the total, not the average.
What the design had to hold
Scope of Work: Feasibility, Mechanical and Structural Design, and a Permit Package
This was an engineering commission from end to end. ACH established whether the dry room could work in this building, designed the mechanical and structural solution to make it work, selected the equipment, and carried the package through the City's permit process.
Feasibility and Mechanical Design
Before any equipment could be chosen the moisture load had to be understood. ACH ran the feasibility study, turned the ingress sources into a number, and sized the mechanical solution to hold minus 25 degrees C against it.
- Feasibility study for the proposed dry room
- Moisture ingress sources identified and quantified
- Dehumidification load calculated to the setpoint
- Rooftop unit and pre-cooling unit selection
- Air distribution designed for 15 air changes per hour
- Temperature held across 20 to 26 degrees C
- ISO Class 8 classification to ISO 14644-1
Structural Design
The mechanical plant had to sit on a roof that was never sized to carry it. ACH designed the reinforcement, detailed the openings the services needed, and checked all of it against the structure that was already standing.
- Joist reinforcement for the rooftop unit
- Joist reinforcement for the pre-cooling unit
- Framing details at each mechanical unit
- New openings detailed in the steel deck
- Loads checked against the existing structure
- Curb and attachment details coordinated
- Structural set issued with the mechanical set
Permit Engineering and City Support
A dry room is a building permit problem as well as an engineering one. ACH took the design through pre-consultation, produced the permit package for the property, and stayed with it until the permit was issued.
- Pre-consultation studies ahead of submission
- Complete permit engineering package produced
- Permit drawings for the works inside the building
- Civil drawings covering the property
- Code review across the affected disciplines
- Responses issued to City review comments
- Design approved and the building permit issued
Where the scope ended. This project was design only. ACH produced the feasibility study, the mechanical and structural design, the equipment selection and the permit package, and supported the submission until the permit was issued. Construction, installation, commissioning and certification were not part of this commission and were not carried out by ACH.
Need the engineering before you can commit to a dry room?
ACH takes battery dry rooms and classified spaces from feasibility study through to an approved permit set, whoever ends up building them.
The Challenges of Designing a Dry Room Inside a Running Plant
The hard part was not the room. It was everything around the room. An existing building, an existing cleanroom that could not be interrupted, a roof that had never been asked to carry this much plant, and a City process running in parallel with the engineering.
A dew point most buildings never see
- 25 C sits far below anything a normally conditioned space holds, and the margin for error in the calculation is small. Every ingress path had to be understood before any equipment could be sized.
A roof not sized for the plant it needed
Dehumidification and pre-cooling had to go somewhere, and the existing roof structure had never been designed to carry them. Reinforcement had to be worked out before the equipment selection could be settled.
An existing cleanroom that stayed running
The dry room had to be designed beside an ISO Class 8 cleanroom with fan filter units already running. Nothing could be proposed that interrupted it, and nothing assumed about the air it was using.
A permit process running in parallel
The design had to satisfy the City as well as the process. Pre-consultation studies, permit drawings for the building and civil drawings for the property were produced and defended in review.
The Solution: Calculate First, Select Second, Carry the Permit
None of this was solved by specifying a bigger dehumidifier. It was solved by doing the arithmetic before the shopping, by treating the running cleanroom as a fixed condition, and by staying with the permit application rather than posting the drawings and hoping.
A moisture balance before any equipment
Every path by which moisture could enter was identified and quantified first. Only then was the dehumidification sized, against a total that had been worked out rather than a figure that felt safe.
The running cleanroom left undisturbed
The existing cleanroom was treated as a fixed condition rather than something to modify. The dry room was designed to sit beside it without disturbing the air it was already using every day.
Structure settled with the equipment
Joist reinforcement and framing were detailed for the rooftop and pre-cooling units, so the structural answer and the equipment selection were resolved together instead of one after the other.
The permit carried, not handed over
Pre-consultation studies, permit drawings for the building and civil drawings for the property were produced and then defended through the City's review, until the permit was finally issued.
The Result: A Design That Passed the City, Not Just the Maths
On a design commission there is no ribbon to cut. The deliverable is a package that somebody else can build from and an authority that agrees it is safe to build. Both were produced here, in nine months, for a room holding a dew point most buildings never see.
-25 °C
Dew point the mechanical design was sized to hold
ISO 8
Classification to ISO 14644-1 at 15 air changes per hour
9
Months from feasibility study to an issued building permit
Permit
Issued on the design package following City review
The building permit was issued on this design. For an engineering commission that is the real test. A load calculation can be argued with and an equipment schedule can be revised, but a permit is an authority looking at the whole package and agreeing it satisfies the code. That is not a formality on a room with these conditions.
What the client holds now is a design that reconciles four things that pull against each other. The dew point sets the dehumidification. The classification sets the air change rate. The equipment sets the structural load. The code sets what the City will accept. Any one of them is manageable alone. Together they are the project.
It began with a feasibility study for a reason. The question on a project like this is not how to build a dry room, it is whether this particular building can hold one at all, and at what cost. Answering that first is cheaper than discovering it later, and it is why the engineering was commissioned ahead of anything else.
What was delivered
Need the engineering before you can commit to the building?
ACH takes dry rooms and classified spaces from the first feasibility study through mechanical and structural design, equipment selection and a permit package, and stays with the submission through review. Whoever ends up building it.